fix: Update _index.md to change loki-stack reference

[rrussell0]

Changes reference to loki-stack to loki as loki-stack is deprecated

What this PR does / why we need it:
Changes reference to loki-stack chart that is deprecated to the correct loki chart

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
Ran into this issue last night when installing Loki for the first time - using the old deprecated chart causes the config to not work.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[trevorwhitney]

@rrussell0 please sign the CLA agreement, thanks for the contribution!

[loki-gh-app]

This PR must be merged before a backport PR will be created.

[loki-gh-app]

This PR must be merged before a backport PR will be created.

[loki-gh-app]

This PR must be merged before a backport PR will be created.

[loki-gh-app]

This PR must be merged before a backport PR will be created.

[JStickler]

Closed via #15074.

[loki-gh-app]

This PR must be merged before a backport PR will be created.